ELEMENTARY MAtiNKTISM. 69 
capable of seizing firmly ou a physical image as a principle, 
of discerning its consequences, and of devising means 
whereby these forecasts of thought may be brought to an 
experimental test. If such a principle be adequate to 
account for all the phenomena if from an assumed cause 
the observed acts necessarily follow, we call the assumption 
a theory, and, once possessing it, we can not only revive at 
pleasure facts already known, but we can predict others 
which we have never seen. Thus, then, in the prosecution 
of physical science, our powers of observation, memory, 
imagination, and inference, are all drawn upon. We 
observe facts and store' them up; the constuctive imagi- 
nation broods upon these memories, tries to discern their 
interdependence and weave them to an organic whole. 
The theoretic principle flashes or slowly dawns upon the 
mind; and then the deductive faculty interposes to carry 
out the principle to its logical consequences. A perfect 
theory gives dominion over natural facts; and even an 
assumption which can only partially stand the test of a 
comparison with facts, may be of eminent use in enabling 
us to connect and classify groups of phenomena. The 
theory of magnetic fluids is of this latter character, and 
with it we must now make ourselves familiar. 
With the view of stamping the thing more firmly on. 
your minds, I will make use of a strong and vivid image. 
In optics, red and green are called complementary colors; 
their mixture produces white. Now I ask you to imagine 
each of these colors to possess a self-repulsive power; that 
red repels red, that green repels green; but that red 
attracts green and green attracts red, the attraction of the 
dissimilar colors being equal to the repulsion of the similar 
ones. Imagine the two colors mixed so as to produce 
white, and suppose two strips of wood painted with this 
white; what will be their action upon each other? Sus- 
pend one of them freely as we suspended our darning- 
needle, and bring the other near it; what will occur? The 
red component of the strip you hold in your hand will 
repel the red component of your suspended strip; but then 
it will attract the green, and the forces being equal, they 
neutralize each other. In fact, the least reflection shows 
you that the strips will be as indifferent to each other as 
two unmagnetized darning-needles would be, under the 
same circumstances. 
